The environmental issues and concerns with respect to smoke stack emissions are well recognized. One of the combustion exhaust gas products that receives considerable attention is fly ash. Much technology and effort has been dedicated to reducing fly ash emissions that are the result of the combustion of hydrocarbonaceous fuel in a combustion unit.
Electrostatic precipitators are one significant type of technology used to reduce fly ash emissions. The basic process used in electrostatic precipitators includes the creation of an electric field in a pipe or passage through which a combustion exhaust gas, including fly ash, flows. When the gas flows through the electric field, particles in the gas (fly ash) pick up a negative charge from the electrons given off by an emitter source. These particles in the gas build up a negative charge and are then attracted to the positive charge on a grounded collector plate. Those particles are then collected there. The fly ash is subsequently collected from the plates by physically rapping the plates and collecting the fly ash that falls off into hoppers where it is then removed.
The efficiency of electrostatic precipitators is affected by several basic factors, one of which is the resistivity of the fly ash particles that the system is trying to collect. For normal operation, the resistivity of the fly ash should lie between about 1×108 and 1×104 Ohm-cm. Values above this range lead to back corona discharge, and below this range lead to re-entrainment of the fly ash back into the exhaust stream because the particles of very low resistivity loose their negative charge very easily. Carbon in the fly ash lowers the resistivity so much that efficient collection in the ESP is impeded. If the particles are highly conductive (i.e., have an excessively low resistivity), then the particles give up there charges very easily and are relatively difficult to retain on a collector plate. An example of this is high carbon content in fly ash, which is known to contribute to electrostatic precipitator inefficiency. On the other hand, very high resistivity particles will retain their charge even after being collected on the collector plates. These high resistivity particles, while initially easy to collect, may form an insulating layer on the collection plates of a system. After a relatively short period of time, the build up of those particles may block the electric flow necessary for the efficient operation of systems.
This build up of high resistivity particles on collector plates also presents other performance problems. One of these problems is referred to as “back-corona” discharge which is a spark or arc across the electric field as a result of the voltage gradient build-up across the collected particle layer on the collector plate. If the electrostatic precipitator voltage becomes too large because of excessively high resistivity of the fly ash (above 109 Ohm cm), gas trapped in this particle layer can ionize and break down, thereby causing a spark or flare that substantially reduces the efficiency of the electrostatic precipitator. Every time this event occurs, there is a “puff” of increased smoke out of the exhaust chimney that is recorded as a transient increase in flue gas opacity. To inhibit this event, ESP controls back off on the potential between the electrodes (reduce the voltage to the electrodes), thereby leading to performance inefficiency and an increase in steady state exhaust opacity.
Particle resistivity can be manipulated and improved by modifying the fuel to be combusted or by modifying the combustion gas before it flows through an electrostatic precipitator. Blending fuels that give off high and low resistivity particles is one way to obtain a desired resistivity in a combustion gas. Alternatively, a combustion gas may be modified or conditioned to make it have the desired resistivity. One of the most recognized methods of modifying or conditioning a combustion exhaust gas is to add sulfur trioxide (SO3) vapor into a combustion exhaust gas stream. The addition of SO3 lowers resistivity. The amount of SO3 can be varied depending on a particular fuel combustion exhaust gas and other operating parameters. Drawbacks of sulfur-type emissions are also recognized, so other types of treatments are desired.